A bimetal thermoswitch employing a ceramic substrate has been proposed in the past as an insulating support for a thermostat (see, for example, Patent Document 1).
FIG. 1A is a side view showing an example of such a bimetal thermoswitch employing a conventional ceramic substrate as an insulating support for a thermostat, FIG. 1B is a top view of the bimetal thermoswitch shown in FIG. 1A, and FIG. 1C is a back view of the bimetal thermoswitch shown in FIG. 1A.
The bimetal thermoswitch comprises a thin and rectangular support 1 made of alumina ceramic, as shown in FIG. 1A, FIG. 1B and FIG. 1C. A groove 2 is formed in the center of the support 1, and both of the longitudinal ends of a basal surface 1a are metalized.
Terminal tabs 3 and 4 are fixed on either of the metalized longitudinal ends of the support 1.
The terminal tabs 3 and 4 have a soldering hole 5 on one end, another end is divided into three parts in a fork-like form, and a pair of protrusions 6 on both sides of the fork and protrusion 7 in the middle are formed in a manner such that they are at different levels. The pair of protrusions 6 at the lower level is joined to the metalized end of the basal surface 1a of the support 1, and the protrusion 7 at the upper level is simply connected to the superior surface of the support 1.
A contact spring 8 has a hole 11 in the approximate center, and a plastic pin 12 is inserted into the hole 11. A head 13 of the pin 12 is caught at the top surface of the contact spring 8, and the lower rod point of the pin 12 penetrates a hole 14 that is provided in the center of a bimetal plate 15 and the groove 2 of the support 1.
The bimetal plate 15 lies between the support 1 and the contact spring 8. A collar 16 of the pin 12 that lies between the contact spring 8 and the bimetal plate 15 serves as a spacer and produces a heat insulating effect between the contact spring 8 and the bimetal plate 15.
A film resistance 17 is placed on the basal plane 1a of the support 1. The film resistance 17 is electrically connected to the terminal tabs 3 and 4 via a conductive strip 18.
When the bimetal plate bends in the opposite direction in response to the higher than switching temperature and lifts up the contact spring 8, electrical current flows only via the film resistance 17, consequently heating the support 1, and the bimetal plate 15 is heated via the support 1. This prevents the bimetal plate 15 from causing a return movement to the initial position where the bimetal plate 15 closes the switch.
As explained above, because the collar 16 of the pin 12 serves as a spacer and produces a heat insulating effect between the contact spring 8 and the bimetal spring 15, the bimetal plate 15 is hardly influenced by Joule heat generated in the contact spring 8.
In the technology in Patent Document 1, it is assumed that the heat source for operating a bimetal thermoswitch (hereinafter referred to as a thermostat) is provided externally, or in other words that the bimetal thermoswitch itself is used as a thermostat, and has a configuration for sensing the external hot air.
However, when a thermostat with a configuration such as that in Patent Document 1 is used for the purpose of the temperature control of a hot plate type heater that has been incorporated into a hair iron or other such device, or for the purpose of the protection of the hot plate type heater by preventing overheating, heat sensing may not function properly due to low thermal responsiveness, and may consequently cause a safety problem.
The thermostat may also be consisted of a large number of components which require many welding and soldering operations to be performed in order to engage each other, and the complex configuration may requires a lot of time and effort for assembly.
Patent Document 1: National Publication of Translated Version No. 63-501833